Add "locktime" option to name_new/firstupdate/update

[JeremyRand]

Is your feature request related to a problem? Please describe.

The send RPC method supports a locktime option. Unfortunately, this is not supported by the name RPC methods.

Describe the solution you'd like

Add the locktime field to the options parameter of the name RPC methods, for parity with currency transactions.

Describe alternatives you've considered

N/A.

Additional context

N/A.

[yanmaani]

Please, merge together all the issues about replicating send's functionality into one master issue. It's an eyesore like this. At least, please consider putting another tag modernize_to_send or something, so I can tell them apart at a glance from e.e. #462.